Manufacturers of injection-molded parts are increasing called upon to provide high quality, complex parts at the lowest possible cost. These demands, in turn, require the development and use of molds and mold tooling capable of producing these complex parts in the most efficient manner possible.
Injection-molded parts with integral sealing surfaces represent one such category of complex and difficult-to-manufacture parts. The sealing surfaces of these parts must provide a durable and reliable liquid or gas-tight seal between mating parts and must typically do so under rigorous conditions of use. Exemplary parts including integral sealing surfaces include caps and closures for food and personal care product containers, automobile headlight housings and enclosures.
The sealing surfaces of these parts typically protrude from, or extend away from, the finished injection-molded part and have a generally curved protruding profile when viewed in side section. The sealing surface is typically formed of a pliant plastic material which is compressed when pressed against the mating part, thereby forming a gasket-like seal between the parts. Certain of these sealing surfaces are referred to in industry as a “crab's claw” seal because of the general resemblance between the appearance of the sealing surface (when viewed in side section) to the profile of a crab's claw.
The tooling utilized to manufacture injection-molded parts including protruding sealing surfaces must include a “negative” surface, or cavity, into which the molten plastic material flows to form the sealing surfaces. Such tooling can be extraordinarily difficult to manufacture because of the difficulty in forming the cavity with the requisite tolerances using conventional forming techniques.
Because conventional technology is unable to provide the requisite high-precision cavity in a single tool, conventional practice has been to use a two-piece core. For example, conventional tooling required to manufacture an injection-molded closure for food and personal care product containers includes a two-piece core and a corresponding cavity in which the core is located. The two-piece core forms the inside surfaces of the closure and the cavity forms the outer closure surfaces.
The two-piece core for forming the protruding sealing surfaces includes (1) the core and (2) an insert part seated in the core. More specifically, the core is machined at one end to provide a female opening including an annular deck having curved walls about the periphery of the opening and a space for receiving a male insert part. The insert part is provided with curved walls about its periphery and is seated in the core opening. The curved walls of the deck and seated insert part form a cavity into which molten plastic material flows to form the sealing surface during the injection-molding process.
Use of a two-piece core carries with it important disadvantages. A two-piece core can unduly prolong each production cycle because the structure of such cores is not optimally conducive to removal of heat energy from the plastic part and mold. The duration of an injection-molding production cycle is dependent on the rate at which the plastic cools after injection into the mold. The mold cannot be opened until the plastic cools sufficiently so as to retain the shape of the molded part.
To accelerate cooling, the core is typically provided with an inner channel or passageway through which a coolant, such as water, is circulated. The coolant removes heat from the core and injection-molded part and facilitates reduction of cycle time.
The coolant passageway cannot extend through the core proximate the distal core end because of the obstruction created by the insert part opening. Any improvement in heat removal would increase the rate of part cooling, decrease production cycle time and reduce manufacturing costs.
Moreover, any requirement that the core include plural parts imposes additional costs on the manufacturer and can lead to manufacture of defective parts if the insert and core are not in complete registry.
The need to remove material in the form of an undercut from metal-containing workpieces and devices is not limited to the tooling industry. Manufacturers of valves, nozzles and other devices can benefit from the use of high-precision undercuts in the manufacture of these types of devices.
It would represent a significant improvement in the art to provide a method of making an undercut and devices including an undercut which would provide the manufacturer with an improved degree of control over device manufacture, which would provide improved devices and which would provide an opportunity for cost control.